HF plasma processes are used, for example, for etching, coating, and ashing of relatively small areas. In plasma processes, however, increasingly larger areas are to be treated, for example, larger wafers in the semiconductor industry, or larger flat-panel displays. While it has so far been relatively easy to produce a homogeneous plasma over the area processed, the dimensions of areas that now need coating or etching extend into the range of the wavelength of the excitation signals of the plasma, or of the harmonics of the excitation signals. Thus, wave-like structures are produced in the plasma, and homogeneous plasma processing becomes difficult.
To counteract wave-like structures, it is known to use several HF plasma generators, which are operated at different or equal frequencies. In U.S. Pat. No. 5,698,062 for example, a HF power generator drive system is disclosed that incorporates a low-voltage source and a frequency divider. The frequency divider drives two HF amplifiers, which can also be considered as HF power generators. The HF amplifiers feed HF power into the plasma process at various points.
U.S. Pat. No. 5,824,606 discloses a method for driving two HF power generators with the same frequency and using these HF power generators to feed HF power into the plasma process at various points. Using a phase-shifter, one adjusts the phase position between the output signals of the HF power generators.
U.S. Pat. No. 6,673,724 B2 describes an arrangement, in which a first signal generator generates a first signal for a first electrode using a DC-voltage supply and a first signal waveform modulator, and a second waveform generator generates a second signal for a second electrode of a plasma chamber using an HF power generator and a second signal waveform modulator.
In experiments for flat panel display manufacturing, medium frequencies (MF) are studied to excite plasma (10 kHz to 1 MHz). To this end, multiple pairs of electrodes are arranged in immediate proximity to further pairs of electrodes. Each pair of electrodes is supplied by a MF generator. The MF generators do not run synchronously and not necessarily with the same frequency.